build: Stop using appstream-util for metainfo validation

Everything, including flathub[0], should be using appstreamcli
now, so stop supporting the old tool as well.

[0] https://docs.flathub.org/blog/improved-build-validation

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3520>

subprojects/extensions-app/data/metainfo/meson.build

@@ -14,14 +14,6 @@ metainfo = i18n.merge_file(
   install_dir: metainfodir
 )
 
-if (appstream_util.found())
-  test('Validating metainfo via appstream_util',
-    appstream_util,
-    args: ['validate', '--nonet', metainfo],
-    workdir: meson.current_build_dir()
-  )
-endif
-
 # Validate Appdata
 if (appstreamcli.found())
   test('Validating metainfo',

subprojects/extensions-app/meson.build

@@ -48,7 +48,6 @@ schemadir = join_paths(datadir, 'glib-2.0', 'schemas')
 servicedir = join_paths(datadir, 'dbus-1', 'services')
 
 gjs = find_program('gjs')
-appstream_util = find_program('appstream-util', required: false)
 appstreamcli = find_program('appstreamcli', required: false)
 desktop_file_validate = find_program('desktop-file-validate', required: false)